1. Field of the Invention
The invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the invention relates to rolling cone rock bits and to an improved cutting structure and cutter element for such bits.
2. Background Information
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by revolving the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or “gage” of the drill bit.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Because drilling costs are typically thousands of dollars per hour, it is thus always desirable to employ drill bits which will drill faster and longer and which are usable over a wider range of formation hardness.
The length of time that a drill bit may be employed before it must be changed depends upon its ability to “hold gage” (meaning its ability to maintain a full gage borehole diameter), its rate of penetration (“ROP”), as well as its durability or ability to maintain an acceptable ROP.
A typical earth-boring bit includes one or more rotatable cone cutters that perform their cutting function due to the rolling movement of the cone cutters acting against the formation material. The cone cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cone cutters thereby engaging and disintegrating the formation material in its path. The rotatable cone cutters may be described as generally conical in shape and are therefore sometimes referred to as rolling cones.
The borehole is formed as the gouging and scraping or crushing and chipping action of the rotary cones remove chips of formation material which are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit. The earth disintegrating action of the rolling cone cutters is enhanced by providing the cone cutters with a plurality of cutter elements. Cutter elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as “TCI” bits, while those having teeth formed from the cone material are commonly known as “steel tooth bits.” In each instance, the cutter elements on the rotating cone cutters break up the formation to form new borehole by a combination of gouging and scraping or chipping and crushing.
The shape and positioning of the cutter elements (both steel teeth and tungsten carbide inserts) upon the cone cutters greatly impact bit durability and ROP and thus, are critical to the success of a particular bit design.
The inserts in TCI bits are typically positioned in circumferential rows on the rolling cone cutters. Most such bits include a row of inserts in the heel surface of the rolling cone cutters. The heel surface is a generally frustoconical surface and is configured and positioned so as to align generally with and ream the sidewall of the borehole as the bit rotates.
Conventional bits typically include a circumferential gage row of cutter elements mounted adjacent to the heel surface but oriented and sized in such a manner so as to cut the corner of the borehole. Conventional bits also include a number of additional rows of cutter elements that are located in circumferential rows disposed radially inward or in board from the gage row. These cutter elements are sized and configured for cutting the bottom of the borehole, and are typically described as inner row cutter elements.
For the most part, inner row inserts in TCI bits have generally been one of two general shapes. One insert typically employed in an inner row may generally be described as a “conical” insert, one having a cutting surface that tapers from a cylindrical base to a generally rounded apex. Such an insert is shown, for example, in FIG. 4A-C in U.S. Pat. No. 6,241,034. Another common shape for an insert for use in inner rows is what generally may be described as a “chisel” shaped. Rather then having the rounded apex of the conical insert, a chisel insert generally includes two generally flattened sides or flanks that converge and terminate in an elongate crest at the terminal end of the insert. The chisel element may have rather sharp transitions where the flanks intersect the more rounded portions of the cutting surface, as shown, for example, in FIGS. 1–4 in U.S. Pat. No. 5,172,779. In other designs, the chisel insert may be contoured so as to eliminate sharp transitions and to present a more rounded cutting surface as shown in FIGS. 3A–D in U.S. Pat. No. 6,241,034. For various applications, the apex in the conventional conical insert and the crest of the conventional chisel insert may be offset from the central axis of the cutter element as shown, for example, in U.S. Pat. No. 4,334,586.
In general, it has been understood that, as compared to a conical inset, the chisel shaped insert provides a more aggressive cutting structure that removes formation material at a faster rate for as long as the cutting structure remains intact. For this reason, in soft formations, chisel shaped inserts are frequently preferred for bottom hole cutting.
Despite this known advantages of chisel shaped inserts, however, such cutters have shortcomings when it comes to drilling in harder formations. In particularly, in hard formations, the relatively sharp cutting edges and corners of the chisel endure high stresses that may lead to chipping and ultimately breakage of the insert. By contrast, conical inserts, having a more rounded and less aggressive shaped cutting surface, withstand harder formations much better than do chisel inserts. Unfortunately, conical inserts suffer from the shortcoming that they are slower to remove formation when drilling in soft formations as compared to a chisel insert. Accordingly, because of these differences, compromises in the cutting structure of a bit typically must be made based on the type of formation expected. Such compromises may be of little significance in the instances where the formations to be encountered are well known. For example, where the interval to be drilled is known to be composed of only soft formation, it is unimportant that a chisel insert could not withstand a harder formation.
Unfortunately, in many locations, the formation hardness cannot be predicted with such certainty. For example, it is common in certain locations to encounter layers of extremely hard rock interspersed within a long interval of relatively soft formation. In these instances, the driller is faced with a difficult problem. Because of their greater speed when drilling in soft formations, it is desirable to use a cutting structure having a chisel shaped inserts; however, when a layer of hard formation is encountered, often at unpredictable depths, the chisel shaped inserts will quickly be ruined such that the bit's ROP will drop dramatically, as for example, from 80 feet per hour to less than 10 feet per hour. Once the cutting structure is damaged and the rate of penetration reduced to an unacceptable rate, the drill string must be removed in order to replace the drill bit. As mentioned, this “trip” of the drill string is extremely time consuming and expensive to the driller.
On the other hand, if the driller were to employ a bit having a cutting structure of conical shaped inserts, a cutting structure that will better survive drilling through the layers of hard formation, the bit's rate of penetration while drilling the soft formation may be intolerably low. As will be understood then, there remains a need in the art for a cutter element and cutting structure that will provide a high rate of penetration when drilling in soft formation, yet be durable enough to withstand encounters with stringers of hard formation, and that will provide an acceptable ROP through both the hard and soft formation.
Another known phenomena detrimental to drill bit life and rate of penetration is a wear phenomena that tends to wear and flatten the cutter element on the side generally facing the borehole wall. As this wear occurs, greater side wall forces are imparted on the bit which tends to lead to bit instability and bit wobble which, in turn, tend to cause the bit to deviate from the intended drilling path and to place greater demands and stresses on the bearings. Furthermore, as the surface of the inserts facing the borehole wall tends to wear toward the center of the insert, the insert becomes sharper and more likely to chip and ultimately to break.
Thus, it would also be desirable to provide a cutter element shaped to resist such off center wear and, when such wear nevertheless does occur, to resist the tendency for the cutter element to break.